Helium 3-helium 4 dilution refrigerator

ABSTRACT

A 3He-4He dilution refrigerator with a continuous, heat exchange of concentrated 3He and dilute 3He streams in parallel flow, in a downward direction.

United States Patent Staas et al.

HELIUM S-HELIUM 4 DILUTION REFRIGERATOR Inventors: Frans Adrianus Staas; Adrianus Petrus Severijns, both of Eindhoven,

Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed: Nov. 6, 1974 Appl. No.: 521,278

Foreign Application Priority Data Nov. 13, 1973 Netherlands 7315487 u.s. Cl 62/502; 62/5 14 Int. Cl. F25B 1/00 Field of Search 62/1 I4, 476, 502, 514

[ Dec. 2, 1975 [56] References Cited UNITED STATES PATENTS 3,58l,5l2 6/l97l Staas et al 62/56 OTHER PUBLICATIONS Wood, M. F.; The He Dilution Refrigerator, Advanced Cryogenics, 1971, N.Y., pp. 245-260.

Primary Examiner-William F. ODea Assistant Examiner-Ronald C. Capossela Attorney, Agent, or Firm-Frank R. Trifari; J. David Dainow [57] ABSTRACT A l-lel-le dilution refrigerator with a continuous, heat exchange of concentrated He and dilute l-le streams in parallel flow, in a downward direction.

7 Claims, 2 Drawing Figures HELIUM 3-HELIUM 4 DILUTION REFRIGERATOR BACKGROUND OF THE INVENTION The invention relates to a helium 3 helium 4 dilution refrigerator for temperatures below the A point of helium. The refrigerator has a supply pipe for a stream of concentrated refrigerated helium 3 which opens into a mixing chamber for helium 3 and helium 4; the mixing chamber is connected by a connecting pipe for a stream of dilute helium 3 to a distillation chamber for separating dilute helium 3 into helium 3 and helium 4, which chamber has an outlet mainly for gaseous helium 3. A continuous heat exchanger is provided which is included in the supply pipe and also in the connecting pipe so as to effect a heat exchange between the concentrated helium 3 stream and the dilute colder helium 3 stream.

A refrigerator of the above-mentioned type, sometimes is referred to as mixing refrigerator is described in Cryogenics, April l966, pages 80-88.

The term continuous heat exchanger is used herein to mean a heat exchanger in which in operation, viewed in the direction of flow, a temperature gradient exists along the heat transmitting partition wall between the two heat exchanging fluids, as distinct from a discrete heat exchanger (step exchanger) in which there is no temperature gradient in the flow direction along the heat transfer partition wall, that is to say in which heat exchange takes place between two discrete temperature levels.

In the operation of the dilution refrigerator concentrated liquid helium 3 is supplied to the mixing chamher which contains helium 3 helium 4. Below 0.87K, phase separation occurs in the liquid helium 3 helium 4 mixture in the mixing chamber, yielding a phase which is rich in helium 3 and behaves as a liquid and a phase which is poor in helium 3 and behaves as a gas. The concentrated helium 3 phase floats on the dilute phase poor in helium 3.

When concentrated helium 3 supplied to the mixing chamber, passes the interface with the dilute helium 3 phase, a cooling effect is produced owing to the large difference between the molar enthalpies of concentrated and dilute helium 3. At the interface liquid helium 3 is as it were evaporated, the heat of evaporation producing the cooling effect. Helium 3 atoms which pass the interface are conveyed via the dilute phase to the distillation chamber of higher temperature owing to the high osmotic pressure of the helium 3 in the dilute solution.

Nonnally the distillation chamber is connected to a pump system. In the distillation chamber the two helium isotopes are separated by'a distillation process. The gaseous phase drawn off is rich in helium 3 (for example 96%) while the dilute liquid solution contains very little helium 3. The substantially pure gaseous helium 3 is condensed and is returned through the supply duct to the upper end of the mixing chamber, so that the cycle is closed.

In order to reduce to a minimum the transport of heat by concentrated helium 3 to the mixing chamber, this concentrated helium 3 during its return is caused to exchange heat with the colder, dilute helium 3 which is flowing from the mixing chamber to the distillation chamber. To realize temperatures of about 0.025K in the mixing chamber a continuous heat exchanger alone is sufficient. If the required temperature is near or even 2 below 0.0l0l(, it is known to include in the supply and connection pipes one or more discrete heat exchanges (sintered copper heat exchangers, thin foil-plate heat exchangers) arranged in series between the continuous heat exchanger and the mixing chamber.

In the known dilution refrigerators the continuous heat exchanger is used as a counterflow heat exchanger in which the concentrated helium 3 and the dilute helium 3 meet in counterflow. This gives rise to problems primarily due to gravity. If the concentrated helium 3 stream runs down and the dilute helium 3 stream runs up, instabilities are produced by the occurrence of convection in the dilute helium 3 stream. In the path from the lower temperature range in the mixing chamber to the higher temperature range in the distillation chamber a temperature gradient occurs in the flow direction in the heat exchange region of the dilute solution. To satisfy the condition of constant osmotic pressure in the superfluid phase the concentration of helium 3 in the dilute solution decreases towards the distillation chamher. A decrease of the helium 3 concentration, however, means an increase in density of the dilute solution, that is to say the dilute solution has a higher specific weight near the distillation chamber than near the mixing chamber. The force of gravity in conjunction with the density gradient gives rise to convection in the dilute solution, so that the condition of constant osmotic pressure is disturbed and flow instabilities involving energy losses occur. If the concentrated helium 3 stream runs up and the dilute helium 3 stream runs down, no disturbing convection is produced in the dilute helium 3 flow.

No difficulties would arise in the concentrated he lium 3 stream if it should consist of pure helium 3. Normally, however, a certain percentage (for example 4%) of helium 4 still is contained in the concentrated helium 3 stream. When this stream is cooled to a temperature below 0.3 to 0.4K in the continuous heat exchanger, phase separation occurs in the said heat exchanger (as in the mixing chamber). By gravity the dilute phase (helium 4) collects in the lower part of the relevant flow channel of the heat exchanger. In analogy with the process in the mixing chamber a cooling effect is produced in this lower part of the heat exchanger channel on the passage of helium 3 through the accumulated dilute superfluid phase, so that the temperature of this channel part falls. As a result, there is substantially no temperature difference anymore between the said channel part containing dilute phase and the other channel part which also contains dilute helium 3 and is in heat exchanging contact with the first mentioned part.

Thus there is substantially no heat transfer between the two channel parts so that the lower part of the heat exchanger is almost inoperative. In addition, the rate of flow of helium 3 may become high in the two dilute 3 He regions, which results in considerable pressure gradients due to the viscous behavior of the helium 3 moving through the superfluid helium. The frictional heat produced thereby also contributes to a reduction of the thermal efficiency of the continuous heat exchanger.

It is an object of the present invention to provide a solution of the above-described problems.

SUMMARY OF THE INVENTION According to the invention the helium 3 helium 4 dilution refrigerator is characterized in that the continuous heat exchanger is arranged in the supply and connection pipes as a parallel flow heat eschanger for the concentrated and dilute helium 3 streams, the continuous parallel flow heat exchanger being positioned in operation so that the two heat exchanging streams run down in a vertical sense. This ensures that both disturbing convection in the dilute helium 3 stream and accumulation of dilute phase (superfluid helium) in the part of the continuous heat exchanger through which concentrated helium 3 flows are prevented.

In an advantageous embodiment of the helium 3- helium 4 dilution refrigerator according to the invention the continuous parallel flow heat exchanger is divided into a plurality of series connected continuous parallel flow heat exchange elements, the heat transferring wall surface area between the concentrated and dilute helium 3 streams of each continuous parallel flow heat exchange element satisfying the relation:

where r: the number of moles of helium 3 which is circulated per second,

T input temperature in K of the concentrated helium 3 flow for the parallel flow heat exchange element,

0 =the Kapitza conduction coefficient of the heattransferring wall defined as where Q heat flux in watts/cm through the heat-transfer ring wall,

T temperature in "K of the concentrated helium 3 stream in the parallel flow heat exchange element,

T temperature in K of the dilute helium 3 stream in the parallel flow heat exchange element.

It was found that thus optimum use of the heat transferring wall surface area between the concentrated and dilute helium 3 streams and hence a heat exchange system of very high thermal efficiency are obtained. For metals the Kapitza conduction coefficient is of the order of 2.5 times W/cm "K and for synthetic materials it is of the order of 17.5 times l0 W/cm K A further advantageous embodiment of the helium 3 helium 4 dilution refrigerator according to the invention is characterized in that each continuous parallel flow heat exchange element comprises two tube elements arranged in coaxial spaced relationship, the wall of the inner tube element forming the heat transfer wall. Having regard to thermodynamic considerations and owing to the comparatively small liquid contents and consequent small heat capacity, the described construction provides the advantage that each continuous parallel flow heat exchange element in itself has a high thermal efficiency.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing of FIG. I which shows schematically and not to scale a helium 3-helium 4 dilution refrigerator having a continuous parallel flow heat exchanger which comprises three series connected elements.

FIG. 2 is another embodiment of the heat exchanger 2a of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, reference numeral 1 denotes a supply pipe which opens in a mixing chamber 2, which chamber is connected by a connection or return pipe 3 to a distillation chamber 4 having an outlet 5. The outlet 5 is connected by a suction pipe 6 to a diffusion pump 7 which in turn is connected to a rotary pump 8. A delivery outlet 9 of the rotary pump 8 is connected by a pipe 10 to the supply pipe 1. The pipe 10 includes heat exchangers 11, 12, I3 and 14 which are accommodated in containers 15, 16, 17 and distillation chamber 4 respectively.

In said containers condensation and further precooling of concentrated helium 3 takes place. For example, the container 15 is filled with liquid nitrogen (78K) while the containers 16 and 17 contain liquid helium of, say, 4.2K and 1.3K respectively. The refrigerator further comprises a continuous parallel flow heat exchanger 20 which is composed of three elements 20a, 20b and 200 which are included in series arrangement in the supply pipe 1 and also in the connection pipe 3. The elements are vertically positioned so that in operation the two streams in the supply and connection pipes both pass through the heat exchange elements downwardly, i.e. in the direction in which the force of gravity acts.

In operation substantially pure gaseous helium 3 delivered to the pipe I0 by the rotary pump 8 is condensed in the heat exchangers B1 to 14 and cooled to a temperature of about O.7K. The condensed concentrated helium 3 is further lowered in temperature in the continuous parallel flow heat exchange elements 20a, b, c and then enters the mixing chamber 2 which contains two phases 22 and 23 of concentrated helium 3 and superfluid dilute helium 3 (helium 3 dissolved in helium 4) respectively which are separated by an interface 21. The passage of helium 3 from the phase 22 via the interface 21 to the phase 23 produces a cooling effect. The helium 3 which has passed the interface 21 is conveyed in the dilute phase through the connecting pipe 3 to the distillation chamber 4 and, during this transport, by parallel flow heat exchange in the continuous heat exchange elements cools the concentrated helium 3 on its way to the mixing chamber 2. In the distillation chamber 4 the dilute helium 3 is separated into helium 3 and helium 4. The substantially pure helium 3 is drawn off through the outlet 5 and the suction pipe 6 by the pump system comprising the diffusion pump '7 and the rotary pump 8, and then is returned to the pipe 10.

The heat transferring wall surface area 0- between the concentrated and dilute helium 3 streams of each of the three parallel flow heat exchange elements 20a, b, c satisfies the relation.

where h the number of moles of helium 3 which is circulated per second, 0 the Kapitza conduction coefficient,

T the input temperature of the concentrated helium 3 stream for the relevant heat exchange element.

The square of the said input temperature appears in the denominator of the right-hand part of the relation. A lower input temperature of the concentrated helium 3 flow permits an increase of the heat transferring wall surface area of the associated heat exchange element. This is expressed in FIG. 1. The temperature of the concentrated helium 3 stream in the supply pipe 1 decreases in the downward direction. The heat transferring wall surface area of the element 20c is greater than that of 20b, which in turn is greater than that of 20a, because the elements have different flow passage lengths.

Since in the parallel flow heat exchanger 20 the concentrated and dilute helium 3 streams both run down in the direction of the force of gravity, the difficulties described hereinbefore are obviated. The construction of the parallel flow heat exchanger from discrete series connected elements having different heat transferring wall surface areas results in a heat transfer system of high thermal efficiency. Preferably the elements each comprise two concentric tubes la and 3a, as shown in FIG. 2, the wall of the inner tube being used for the heat transfer between the two streams. Obviously the continuous parallel flow heat exchanger may be divided into two or more than three elements while satisfying the above relation.

Condensation and precooling of the concentrated helium 3 may be effected by means other than a bath of liquid nitrogen and two baths of liquid helium, and also other pump systems, which may or may not operate at room temperature, may be used. If desired, further discrete heat exchangers (made for example of sintered copper) may be included in the supply and connection pipes l and 3 respectively between the continuous heat exchange elements 200 and the mixing chamber 2.

What is claimed is:

1. In a He He dilution refrigerator for temperatures below the A point of helium, including first means for condensing and cooling gaseous helium into con- .centrated liquid He, a mixing chamber containing con- 'centrated He and superfluid dilute He, a distillation chamber for separating dilute He into He and He, and a supply duct for flowing concentrated liquid He from said first means to said mixing chamber, anda return duct for flowing dilute He from said mixing chamher to said distillation chamber, the improvement in combination therewith of a continuous heat exchanger comprising adjacent and parallel, concurrent oriented lportions of said supply and return duct in heatexchange relationship.

2. A refrigerator according to claim 1 wherein said heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements, each element having a heat-transferring wall surface-area 0' between the concentrated He in the supply duct and the dilute He in the return duct, said 0' satisfying the relation:

where it the number of moles of helium 3 which is circulated per second,

T input temperature in K of the concentrated helium 3 stream for the parallel flow heat exchange element,

0 the Kapitza conduct coefficient of the heat transfer wall, defined as where Q heat flux in Watts/cm through the heat transferring wall,

T temperature in "K of the concentrated helium 3 stream in the parallel flow heat exchange element,

and

T temperature in K of the dilute helium 3 stream in the parallel flow heat exchange element.

3. A refrigerator according to claim 2 wherein each of said heat-exchange elements comprises inner and outer tubes in coaxial relationship with a bore through the inner tube and an annular space between said tubes, said bore and annular space corresponding to said supply and return ducts and the wall of the inner tube forming said heat-transfer wall.

4. A refrigerator according to claim 1 wherein said first means comprises a heat exchanger for reducing the temperature of the helium to approximately l.3K.

5. A refrigerator according to claim 4 wherein said first means comprises three heat-exchangers in series containing liquefied gases at temperatures of approximately 78K, 4.2K, and 1.3K.

6. A refrigerator according to claim 1 wherein said continuous heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements for cooling to successively lower temperatures, and said return duct provides a flow path from said mixing chamber first to the heat exchanger element having the lowest of said temperatures, and then to heat exchanger elements having successively higher temperatures.

7. A He He dilution refrigerator operable with a supply of gaseous helium, comprising, pump means for circulating said helium, first means for cooling and condensing said helium from said pump means, a continuous heat exchanger for further cooling said liquid helium into concentrated He, a mixing chamber containing dilute He and receiving said concentrated He with an interface between said concentrated and dilute He and resulting refrigeration, a supply duct for flow ing said liquid He from said first means through said heat exchanger to said mixing chamber, a return due for flowing said dilute He from said mixing chamber through said continuous heat exchanger where it provides refrigeration for said liquid He flowing there through, and thence to said mixing chamber, a distilla tion chamber for separating gaseous He from said di lute liquid He received via said return duct from saic heat exchanger, and a suction pipe for flowing said gas eous He from said distillation chamber to said purnr means, said heat exchanger comprising portions of sait return duct and said supply duct which are adjacen and situated in heat transfer relationship for paralle flow in a concurrent direction. 

1. IN A 3HE - 4HE DILUTION REFRIGERATION FOR TEMPERATURES BELOW THE $ POINT OF HELIUM, INCLUDING FIRST MEANS FOR CONDENSING AND COOLING GASEOUS HELIUM INTO CONCENTRATED LIQUID 3HE, A MIXING CHAMBER CONTAINING CONCENTRATED 3HE AND SUPERFLUID DILUTE 3HE, A DISTILLATION CHAMBER FOR SEPARATING DILUTE 3HE INTO 3HE AND 4HE, AND A SUPPLY DUCT FOR FLOWING CONCENTRATED LIQUID 3HE FROM SAID FIRST MEANS TO SAID MIXING CHAMBER, AND A RETURN DUCT FOR FLOWING DILUTE 3HE FROM SAID MIXING CHAMBER TO SAID DISTILLATION CHAMBER, THE IMPROVEMENT IN COMBINATION, THEREWITH OF A CONTINUOUS HEAT EXCHANGER COMPRISING ADJACENT AND PARALLEL, CONCURRENT ORIENTED PORTIONS OF SAID SUPPLY AND RETURN DUCT IN HEAT-EXCHANGE RELATIONSHIP.
 2. A refrigerator according to claim 1 wherein said heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements, each element having a heat-transferring wall surface-area sigma between the concentrated 3He in the supply duct and the dilute 3He in the return duct, said sigma satisfying the relation:
 3. A refrigerator according to claim 2 wherein each of said heat-exchange elements comprises inner and outer tubes in coaxial relationship with a bore through the inner tube and an annular space between said tubes, said bore and annular space corresponding to said supply and return ducts and the wall of the inner tube forming said heat-transfer wall.
 4. A refrigerator according to claim 1 wherein said first means comprises a heat exchanger for reducing the temperature of the helium to approximately 1.3*K.
 5. A refrigerator according to claim 4 wherein said first means comprises three heat-exchangers in series containing liquefied gases at temperatures of approximately 78*K, 4.2*K, and 1.3*K.
 6. A refrigerator according to claim 1 wherein said continuous heat exchanger comprises a plurality of series-connected, continuous, parallel-flow heat exchanger elements for cooling to successively lower temperatures, and said return duct provides a flow path from said mixing chamber first to the heat exchanger element having the lowest of said temperatures, and then to heat exchanger elements having successively higher temperatures.
 7. A 3He - 4He dilution refrigerator operable with a supply of gaseous helium, comprising, pump means for cirCulating said helium, first means for cooling and condensing said helium from said pump means, a continuous heat exchanger for further cooling said liquid helium into concentrated 3He, a mixing chamber containing dilute 3He and receiving said concentrated 3He, with an interface between said concentrated and dilute 3He and resulting refrigeration, a supply duct for flowing said liquid 3He from said first means through said heat exchanger to said mixing chamber, a return duct for flowing said dilute 3He from said mixing chamber through said continuous heat exchanger where it provides refrigeration for said liquid 3He flowing therethrough, and thence to said mixing chamber, a distillation chamber for separating gaseous 3He from said dilute liquid 3He received via said return duct from said heat exchanger, and a suction pipe for flowing said gaseous 3He from said distillation chamber to said pump means, said heat exchanger comprising portions of said return duct and said supply duct which are adjacent and situated in heat transfer relationship for parallel flow in a concurrent direction. 